Which code can help me to calculate a time series average for the defined regions?

Hey team, I want to calculate a time series average for the assigned regions (Northern , Eastern , southern and central as shown in the map) for this map, using a 6x6 cell data as shown in the excel sheet(ignore the first column and last row to suit the shape read). Kindly help me. thanks

Please refer to excel sheet for clarity. I will be glad for any useful help. Thanks